Photomechanical emulsions containing rare-earth metal salts



United States Patent C) PHQTOh/IECHANICAL EMULSIONS CUNTAINING This invention relates to photograpihc silver halide emulsions and, more particularly, to the manufacture of improved emulsions suitable for half-tone and line reproduction.

The photographic emulsions Which are used for pictorial purposes by professional and amateur photographers are designed to produce upon development a medium or low-contrast image, whereas emulsions used for halftone and line reproduction in the photo-mechanical and lithographic field must produce an extremely contrasting image so as to satisfy the critical standards of the lithographer and photoengraver.

The gradation of an emulsion and the film of which it is part is customarily defined by a graphic analysis of the response to light shown by the film under certain definite and accurately controlled conditions of exposure and development. For this purpose, the film is exposed in a sensitometer and developed under controlled and standardized conditions. The resulting densities are measured and plotted on a graph against the exposure values which produce them. The line drawn through the resulting points is termed the characteristic curve or the H & D curve for that particular sample. Inasmuch as most photomechanical processes require a film of extremely high contrast, optimum results are obtained only when all of the exposed portions of the negative have the same high density and when the unexposed portions of the film are completely clear. Thus, the characteristic curve of an ideal graphic arts film would consist of a horizontal line which is met at a sharp point by the straight and steep line portion of the characteristic curve. However, no such ideal conditions are possible, and the characteristic curve of a graphic arts film has always an extended toe portion of intermediate density. From a practical point of view, this intermediate density range exerts an undesirable effect on the half-tone image because it contributes toward an undesirable fuzziness around the dark edges of half-tone image dots.

I have now discovered that the quality of high-contrast emulsions used in the graphic arts field can be improved by adding to the emulsions during their preparation a Water-soluble rare-earth salt, particularly a water-soluble cerous salt. Such photosensitive emulsions, the films prepared therefrom, and the methods of preparing such emulsions constitute the purposes and objects of this invention.

The gelatin silver halide emulsions used in the preparation of graphic art films are unsensitized, or-thochromatic or panchromatic, high-contrast emulsions. When these emulsions are developed with the so-called infectious type developers which contain, in addition to a developing agent of the hydroquinone type, a mixture of paraiormaldehyde and potassium metabisulfite or the reaction prod not of an aldehyde and sodium bisulfite as described in United States Patent 2,313,523, they yield images which have a gamma or gradient which exceeds 5.0. This gradient may be as high as 15 although in most instances the preferred range is between 6.0 and 14.0. The silver halide used in the preparation of these emulsions issilver chloride, silver chlorobromide or silver bromide. These emulsions should be free of iodide, although small concentrations of iodide can be tolerated so long as they do 3,173,289 Patented Apr. 13, 1965 "ice not exceed 0.5 percent of iodide based on the amount of silver nitrate used in the preparation of the silver halide emulsions. When silver chlorobromide emulsions are used, the relative amounts of halides generally range from 2 to 8 parts of chloride per part of bromide.

The amount of rare-earth metal used ranges from 0.75 gram to 20.0 grams per 1,000 grams of silver nitrate used in the preparation of the emulsion; the preferred range rs between 1.5 grams to 5.0 grams per 1,000 grams of silver nitrate. Expressed in terms of ions or metal content, the amount of rare-earth ion used ranges from about 1 to 20 parts per 1,000 parts of silver ion. Expressed in terms of coated area, the amounts of rare-earth metal present in the finished and coated emulsion range from 5 to milligrams per square meter whereas the amount of silver present in the finished coated emulsion ranges from 2 to 8 grams per square meter. When used in the concentration specified, the rare-earth salt, including the cerous salt, produces a marked increase in contrast and a shortening in the toe portion of the emulsion. As a result, the graphic art films produced from these emulsions, which are used in the half-tone and line processes, give much cleaner and darker dots which are sharp around the edges, whereas the dots prepared from conventional emulsions are dull and fuzzy those leading to an inferior image.

In the preparation of the emulsions, the rare-earth salt is added in the form of a water-soluble salt. It can be mixed with the alkali halide solution which contains gelatin as the colloidal carrier, or it can be added to the silver nitrate solution to produce silver halide crystals or grains which contain minute amounts of rare-earth impurities embedded in the grain structure. However, the rare-earth salt can also be added, completely or in part, at any other stage of the emulsion preparation; namely, after the silver precipitation, during the first ripening (Ostwald), or during the after-ripening step. A noticeable synergistic eiiect is obtained by adding the rare-earth salts in portions during the difierent steps of the emulsionmaking process.

Among the rare-earth salts suitable in carrying out the invention are the following: 1

Cerous acetate, Ce(CH COO) Cerous chloride, CeCl Cerous bromide, CeBr Cerous nitrate, Ce(NO Cerous sulfate, Ce (SO Gadolinium acetate, Ga(CH COO) Gadolinium chloride, GaCl Gadolinium nitrate, Ga(NO Lanthanum acetate, La(CH COO) Lanthanum chloride, LaCl Lanthanum nitrate, La(NO Neodymium acetate, Nd (CH COO) Neodymium bromide, NdBr Neodymium chloride, NdCl Neodymium nitrate, Nd(NO Neodymium sulfate, Nd (SO Yttrium bromide, YBr,

Yttrium acetate, Y(CH COO) Yttrium chloride, YCl

Samarium-gadolinium chloride, SmCl GdCl Rare-earth salts such as cerous chloride have been used previously in the manufacture of photographic products :to the extent of adding them to photographic papers in order to reduce the retention of the sodium thiosulfate by the barium sulfate layer. However, this use produced no special effects as far as the sensitivity and gradation of the emulsion were concerned. 7

Without wishing to be bound by or limited to any particular theory for the mechanism involved in producing the increased contrast and the shortened toe portion, it is believed that the rare-earth ion is built into the host silver halide lattice as an impurity occupying lattice sites throughout the crystal normally occupied by silver ions, resulting in a distortion of the normal lattice structure and thereby altering the contrast of the emulsion.

The following examples serve to illustrate the invention, but it is to be understood that the invention is not to be restricted thereto.

Example Two solutions were prepared as follows.

Solution I:

Water milliliters 815.0 Potassium bromide grams 8.0 Sodium chloride do 24.0 Gelatin -do 40.0 Cerous chloride do 0.13

Solution II:

Water milliliters 135.0 Silver nitrate grams 67.5

At a temperature range of 50 to 70 C., Solution II is added to Solution I within three minutes. After the addition has been completed, 20 grams of dry gelatin is added to the silver chloride emulsion and the whole mixture digested for about 45 minutes. The emulsion is then chilled, shredded and washed to remove all water-soluble salts. The emulsion is re-liquefied by heating, brought to a temperature of 50 C. and held there for about 25 minutes. Suitable coating finals including a hardener and a wetting agent are added, and the emulsion is coated onto a cellulose acetate film base. After exposure in a Type HB Sen-sitometer, the exposed film was developed for three minutes in the following lithographic developer which is described on page 38 of the Ansco Graphic Arts Handbook (1958):

Water 2,000.0 Sodium sulfite, anhydrous ..grams 120.0 Paraformaldehyde do... 30.0 Potassium metabisulfite do 10.5 Boric acid (crystals) do 30.0 Hydroquinone do 90.0 Potassium bromide do 6.0 Water to make 4.0 liters.

Example II Example I was repeated with the exception that the cerous chloride was omitted from Solution I and that 0.25 gram of cerous nitrate was added to Solution II. The material was digested, ripened and coated in the manner described in Example I and then developed in the formaldehyde bisulfite containing developer described on page 369 of Henney and Dudleys Handbook of Photography. The characteristic curve of this material showed a much shorter toe portion and a much steeper gradation than a type material which had been produced without the use of cerous nitrate.

4 Example III Two solutions were prepared as follows: Solution I:

Water ccs.. 820.0

Potassium bromide "grams" 5.0

Sodium chloride do.. 30.0

Gelatin do 45.0

Solution H:

Water milliliters 130.0

Silver nitrate grams 67.9

The two solutions were first heated to a temperature of 65 C. and then mixed by adding Solution II to Solution I at a fast rate over a period of four minutes with constant agitation. A solution of 0.25 gram of cerous nitrate in 10 milliliters of water is added to the emulsion followed by 30 grams of dry gelatin. The mixture is maintained at 65 C. for 60 minutes with agitation. The emulsion was then chilled, shredded and thoroughly washed. The emulsion was then rewarmed, brought to 60 C. and held there for '30 minutes. Saponin, an orthochromatic sensitizing dye, and a hardener were added; and the emulsion coated onto a cellulose acetate support and dried. The film base obtained was exposed with are light in a Type 1113 Sensitometer and developed in a graphic arts developer of the commercial formaldehyde-bisulfite type sold under the name of Ansco Reprodol. The developed film was short-stopped, fixed and washed. The characteristic curve of this material had a gradient of 9.0 and a maximum density of 5.5, whereas a type material prepared without the cerous salts had a gradient of 7.0 and a maximum density of 4.8. The toe portion of the material prepared with the cerous salt was considerably shorter than that of the type material. As a result, the dot structure of a practical half-tone print was considerably sharper around the edges and cleaner than that of a type material prepared without the cerous nitrate.

Example IV 'Example I was repeated with the exception that the cerous chloride was omitted from Solution I but added just prior to the after-ripening step. The emulsion was Example V Two solutions were prepared as follows.

Solution I:

Water .milliliters 800.00 Potassium bromide grams 6.00 Sodium chloride do.. 18.00 Gelatin do 40.00 Neodymium chloride do 0.25

Solution II:

Water milliliters 135.0 Silver nitrate grams 50.7

The two solutions were first heated separately to a temperature of 60 C. and then mixed by the addition of Solution II to Solution I over. a period of 3 minutes with constant agitation. Thereafter, 20 grams of dry gelatin were added, and the mixture was maintained at 60 C. for 45 minutes with agitation. The emulsion was then chilled, thoroughly washed with water, remelted and afterripened for 25 minutes at 60 C. Saponin was added as a sni'eese Wetting agent; formaldehyde was added as a hardener, and the emulsion coated onto a cellulose ester base and dried.

After exposure in a Type 113 Sensitometer, the film was developed for three minutes at 20 C. in a high-contrast Ansco Reprodol Developer together with a type film which had been prepared without the use of the rareearth salt.

The emulsion prepared with the addition of the neodymium salt had a higher contrast and a shorter toe region than a type emulsion prepared without the neodymium salt. In a practical test, the dot structure was sharper and cleaner than that of the type.

Example Vl Example V was repeated with the exception that the 0.25 gram of neodymium chloride was replaced by ,02 gram of lanthanum nitrate. The improvement in contrast and dot structure obtained was of the same order as that described in Example IV.

While the present invention has been described with reference to certain preferred procedures, it is evident that the invention is not limited thereto, that variations may be made to the procedures described therein and that equivalent materials may be substituted without departing from the scope of the invention. For instance, silver chloride emulsions can be used in place or" the silver chlorobromide emulsions illustrated in the examples. In place of the specific rare-earth salts illustrated by the examples, other rare-earth salts such as water-soluble yttrium and gadolinium salts can be used with similar results. Synthetic colloidal carrier materials such as polyvinyl alcohol or partially hydrolyzed polyvinyl acetate can be used in place of gelatin. Special sensitizing agents which are known to the art can be added to the emulsions to increase their inherent speed, -particularly such sensitizing agents which render the emulsion more sensitive to the blue, green and red regions of the spectrum. In place of saponin, synthetic surfactants or wetting agents such as isopropyl-naphthalene sulfonic acid or other cationic, anionic and non-ionic wetting agents can be used which aid in the preparation of even coatings without bubble formation. In place of the cellulose ester film base, other film supports such as polystyrene, polyvinyl chloridepolyvinyl acetate or polycarbonates may be used.

It will be understood, therefore, what I contemplate as included within my invention all modifications and equivalents falling within the scope of the appended claims.

I claim:

1. In a process of preparing a photographic image in a photographic gelatin silver halide emulsion layer, capable of forming, upon exposure and development in an infectious type developer solution, containing in an aqueous solution, hydroquinone, as the sole developing agent, a strong inorganic base and a compound of an aldehyde with a bisulfite, a high contrast image of the type suitable for half-tone reproduction as distinguished from medium and low contrast emulsions and having a gamma higher than 5 .0 when developed in said developer, which includes preparing said emulsion by mixing an aqueous silver nitrate solution with an aqueous alkali halide solution having an iodide content corresponding to less than 0.5 percent of iodine based on the amount of silver nitrate used in the preparation of the emulsion, in the presence of gelatin which serves as the colloidal carrier material, ripening, Washing and after-ripening the gelatin emulsion thus formed, the improvement which comprises adding to the emulsion at a stage prior to the after-ripening step, a water-soluble rare-earth metal salt selected from the group which consists of the water-soluble salts of cerium,

6 further increasing the gamma and shortening the toe portion of the characteristic curve of said emulsion.

2. In a process of preparing a photographic image in a photographic gelatin silver halide emulsion layer, capable of forming upon exposure and development in an infectious type developer solution containing in an aqueous solution hydroquinone as the sole developing agent, a strong inorganic base and a compound of an aldehyde with a bisulfite, a high contrast image of the type suitable for half-tone reproduction as distinguished from medium and low contrast emulsions and having a gamma higher than 5.0 when developed in said developer, which includes preparing said emulsion by reacting an aqueous silver nitrate solution with an aqueous alkali metal halide solution in the presence of gelatin which serves as the colloidal carrier material so as to form a dispersion of silver halide grains in said gelatin, the improvement which comprises increasing the contrast and shortening the toe portion of the characteristic curve of said emulsion by mixing said two solutions in the presence, in at least one of said solutions, of a Watersoluble rare-earth metal salt, selected from the group consisting of cerium, gadolinium, lanthanum, neodymium, yttrium and samarium in a concentration of the rare-earth metal salt ranging from 0.75 gram to 20.0 grams per 1,000 grams of silver nitrate used in the preparation of the emulsion.

3. In a process of preparing a photographic image in a photographic gelatin silver halide emulsion layer, capable of forming upon exposure and development in an infectious type developer solution containing in an aqueous solution hydroquinone as the sole developing agent, a strong inorganic base and a compound of an aldehyde with a bisulfite, a high contrast image of the type suitable for half-tone reproduction as distinguished from medium and low contrast emulsions and having a gamma higher than 5.0 when developed in said developer, which process includes preparing an emulsion in which the silver halide has a silver iodide content corresponding to less than 0.5 percent of iodine based on the amount of silver nitrate used in the preparation of the emulsion, and wherein said process includes mixing an aqueous silver nitrate solution with an aqueous alkali halide solution containing gelatin as the colloidal carrier material, ripening, washing and after-ripening the emulsion thus formed, the improvement which comprises carrying out said after-ripening step in the presence of a water-soluble rare-earth metal salt selected from the group consist ing of the water-soluble salts of cerium, gadolinium, lanthanum, neodymium, yttrium and samarium, which is present in the emulsion in a concentration ranging from 0.75 gram to 20.0 grams of rare-earth metal salt per 1,000 grams of silver nitrate used in the preparation of said emulsion, thereby further increasing the gamma of said emulsion.

4. A process according to claim 1, wherein a part of said rare-earth metal salt is added during the mixing step and another part of said rare-earth metal salt is added after the washing step and prior to the after-ripening step.

5. A process according to claim 1 wherein said rareearth metal salt is a water-soluble cerous salt.

6. A process according to claim 2 wherein said rare earth metal salt is a Water-soluble cerous salt.

7. A process according to claim 3 wherein said rareearth metal salt is a water-soluble cerous salt.

8. A rocess accordin to claim 4 wherein said rareearth metal salt is a water-soluble cerous salt.

9. A process according to claim 2 wherein said rareearth metal salt is cerous chloride and the cerous chloride (References on following page) References Cited by the Examiner UNITED STATES PATENTS Smith et a1. 96110 Russell 96-110 Jones 96110 Jones 96-110 De Pauw 9694 Dann et a1 96-94 FOREIGN PATENTS 9/38 Great Britain.

OTHER REFERENCES Shono etal; Photographic Abstracts, vol. 36, No. 3, page 138, October 31,1956.

Stookey: Industrial Engineering Chemistry, vol. 41, pp. 856-861 (1949). NORMAN G. TORCHIN, Primary Examiner. MILTON STERMAN, Examiner. 

1. IN A PROCESS OF PREPARING A PHOTOGRAPHIC IMAGE IN A PHOTOGRAPHIC GELATIN SILVER HALIDE EMULSION LAYER, CAPABLE OF FORMING, UPON EXPOSURE AND DEVELOPMENT IN AN INFECTIOUS TYPE DEVELOPER SOLUTION, CONTAINING IN AN AQUEOUS SOLUTION, HYDROQUINONE, AS THE SOLE DEVELOPING AGENT, A STRONG INORGANIC BASE AND A COMPOUND OF AN ALDEHYDE WITH A BISULFITE, A HIGH CONTRAST IMAGE OF THE TYPE SUITABLE FOR HALF-TONE REPRODUCTION AS DISTINGUISHED FROM MEDIUM AND LOW CONTRAST EMULSIONS AND HAVING A GAMMA HIGHER THAN 5.0 WHEN DEVELOPED IN SAID DEVELOPER, WHICH INCLUDES PREPARING SAID EMULSION BY MIXING AN AQUEOUS SILVER NITRATE SOLUTION WITH AN AQUEOUS ALKALI HALIDE SOLUTION HAVING AN IODIDE CONTENT CORRESPONDING TO LESS THAN 0.5 PERCENT OF IODINE BASED ON THE AMOUNT OF SILVER NITRATE USED IN THE PREPARATION OF THE EMULSION, IN THE PRESENCE OF GELATIN WHICH SERVES AS THE COLLOIDAL CARRIER MATERIAL, RIPENING, WASHING AND AFTER-RIPENING THE GELATIN EMULSION THUS FORMED, THE IMPROVEMENT WHICH COMPRISES ADDING TO THE EMULSION AT A STAGE PRIOR TO THE AFTER-RIPENING STEP, A WATER-SOLUBLE RARE-EARTH METAL SALT SELECTED FROM THE GROUP WHICH CONSISTS OF THE WATER-SOLUBLE SALTS OF CERIUM, GADOLINIUM, LANTHANUM, NEODYMIUM, YTTRIUM AND SAMARIUM IN A CONCENTRATION RANGING FROM 0.75 GRAM TO 20.0 GRAMS OF RARE-EARTH METAL SALT PER 1,000 GRAMS OF SILVE NITRATE USED IN THE PREPARATION OF SAID EMULSION, THEREBY FURTHER INCREASING THE GAMMA AND SHORTENING THE TOE PORTION OF THE CHARACTERISTIC CURVE OF SAID EMULSION. 